It is well known in the art that heat will destroy microorganisms. The presence of moisture accelerates this destruction by denaturing or coagulation of the proteins making up the microorganisms. Most microorganisms contain sufficient water so that moderate heat alone, e.g. 80.degree.-100.degree.C., will destroy the microorganism. Many bacterial spores, on the other hand, contain substantially no water and require elevated temperatures in excess of 150.degree.C. for their destruction where dry heat is used. Hence, the destruction of such organisms is generally carried out in the presence of steam in autoclaves.
Such steam sterilization is generally carried out at temperatures of about 250.degree.F. for at least 12 to 15 minutes or for shorter times at higher temperatures e.g. 270.degree.F. Often, to insure a sufficient safety margin, times as long as 30 minutes are used. Such long sterilization times give the operator a greater degree of confidence that steam has penetrated throughout the autoclave and among all of its contents. However, such long heat cycles are disadvantageous from the standpoint of economy of time, energy consumption, and severe shortening of the useful life of the sterilized material, e.g., fabric gowns, drapes, muslim products etc.
From time to time attempts have been made to develop sterilization indicators which permit quality control of sterilization with the confidence that all microorganisms have been destroyed. Presently the most satisfactory method is the use of spore strips. Spores which are particularly difficult to destroy are selected as the control standard, e.g., Bacillus Subtilis var. Niger and Bacillus Stearothermophilus. The spore strip is placed in the autoclave with the materials to be sterilized. At the end of the sterilization cycle, the spore strip is studied to determine whether it is possible to grow organisms in a suitable culture medium. Failure of the spores to reproduce indicates death of spores; and hence, adequate sterilization.
Although this control technique is accurate, it suffers from several inherent disadvantages, (1) excessive cost (2) delay between processing and control data (3) batch to batch variation of the spores and (4) heat resistance of spores decreases with storage time.
Several attempts have been made to devise chemical type sterility indicators. The crudest variety is a product known as Temp-Tube, see for example, U.S. Pat. Nos. 3,313,266; 3,341,238; and 3,652,249. The device consists of nothing more than a sealed tube containing a compound with a melting point which corresponds to the sterilization temperature. The device is capable of doing no more than indicating whether or not the autoclave was held at a temperature above or below the melting point for a period of time once the melting point is reached. Hence, the device only indicates that the desired melting point temperature was reached for a period of time sufficient to melt the indicator.
Other sterility indicators rely on a temperature accelerated chemical reaction to cause color change in an indicator. Though some of these devices purport to be operative at more than one temperature/time condition, they suffer from the disadvantage that they do not match the spore kill temperature/time relationships. The thermal resistance of spores of a particular species at any temperature is characterized by its temperature coefficient. The symbol Q.sub.10 is used to designate the temperature coefficient over a range of 10.degree.C. It means the ratio of the death rate constant at a particular temperature to the death rate constant at a temperature 10.degree.C. lower. Generally, the measurements are made for a fixed time interval, e.g., 9 minutes. If the constants at any two temperatures, t.sub.1, and a temperature 10.degree.C. higher, t.sub.2, are known, then Q.sub.10 may be calculated from the equation: ##EQU1## wherein t.sub.1 and t.sub.2 are as defined and K.sub.1 and K.sub.2 are the respective death rate constants. Spores generally exhibit a Q.sub.10 value of about 10. Therefore, it is desirable to have a sterility indicator which will, in a sense, mimic spore kill. To do so, the ratio of the effect of temperature as a function of time on a measurement taken at one temperature as compared to the same measurement at another temperature 10.degree.C. lower should also be 10. To be useful as a sterility indicator, this relationship must also be dependent on the presence of moisture, since the spore kill time/temperature relationship is vastly different in the dry or wet state. In the absence of moisture spore kill at 250.degree.- 270.degree.F. is negligible, but in the presence of steam spore kill is virtually complete for the most resistant strains at these temperatures in about 12-2 minutes.